This invention relates to water heaters, particularly to improvements to gas fired water heaters adapted to render them safer for use and to reduce NOx emissions.
The most commonly used gas-fired water heater is the storage type, generally comprising an assembly of a water tank, a main burner to provide heat to the tank, a pilot burner to initiate the main burner on demand, an air inlet adjacent the burner near the base of the jacket, an exhaust flue and a jacket to cover these components. Another type of gas-fired water heater is the instantaneous type which has a water flow path through a heat exchanger heated, again, by a main burner initiated from a pilot burner flame.
For convenience, the following description is in terms of storage type water heaters but the invention is not limited to this type. Thus, reference to xe2x80x9cwater container,xe2x80x9d xe2x80x9cwater containment and flow means,xe2x80x9d xe2x80x9cmeans for storing or containing waterxe2x80x9d and similar such terms includes water tanks, reservoirs, bladders, bags and the like in gas-fired water heaters of the storage type and water flow paths such as pipes, tubes, conduits, heat exchangers and the like in gas-fired water heaters of the instantaneous type.
A particular difficulty with many locations for water heaters is that the locations are also used for storage of other equipment such as lawn mowers, trimmers, snow blowers and the like. It is a common procedure for such machinery to be refueled in such locations.
There have been a number of reported instances of spilled gasoline and associated extraneous fumes being accidently ignited. There are many available ignition sources, such as refrigerators, running engines, electric motors, electric and gas dryers, electric light switches and the like. However, gas water heaters have sometimes been suspected because they often have a pilot flame.
Vapors from spilled or escaping flammable liquid or gaseous substances in a space in which an ignition source is present provides for ignition potential. xe2x80x9cExtraneous fumes,xe2x80x9d xe2x80x9cfumesxe2x80x9d or xe2x80x9cextraneous gasesxe2x80x9d are sometimes hereinafter used to encompass gases, vapors or fumes generated by a wide variety of liquid volatile or semi-volatile substances such as gasoline, kerosene, turpentine, alcohols, insect repellent, weed killer, solvents and the like as well as non-liquid substances such as propane, methane, butane and the like.
Many inter-related factors influence whether a particular fuel spillage leads to ignition. These factors include, among other things, the quantity, nature and physical properties of the particular type of spilled fuel. Also influential is whether air currents in the room, either natural or artificially created, are sufficient to accelerate the spread of fumes, both laterally and in height, from the spillage point to an ignition point yet not so strong as to ventilate such fumes harmlessly, that is, such that air to fuel ratio ranges are capable of enabling ignition are not reached given all the surrounding circumstances.
One surrounding circumstance is the relative density of the fumes. When a spilled liquid fuel spreads on a floor, normal evaporation occurs and fumes from the liquid form a mixture with the surrounding air that may, at some time and at some locations, be within the range that will ignite. For example, the range for common gasoline vapor is between about 2% and 8% gasoline with air, for butane between 1% and 10%. Such mixtures form and spread by a combination of processes including natural diffusion, forced convection due to air current drafts and by gravitationally affected upward displacement of molecules of one less dense gas or vapor by those of another more dense. Most common fuels stored in households are, as used, either gases with densities relatively close to that of air (e.g. propane and butane) or liquids which form fumes having a density close to that of air, (e.g. gasoline, which may contain butane and pentane among other components is very typical of such a liquid fuel).
In reconstructions of accidental ignition situations, and when gas water heaters are sometimes suspected and which involved spilled fuels typically used around households, it is reported that the spillage is sometimes at floor level and, it is reasoned, that it spreads outwardly from the spill at first close to floor level. Without appreciable forced mixing, the air/fuel mixture would tend to be at its most flammable levels close to floor level for a longer period before it would slowly diffuse towards the ceiling of the room space. The principal reason for this observation is that the density of fumes typically involved is not greatly dissimilar to that of air. Combined with the tendency of ignitable concentrations of the fumes being at or near floor level is the fact that many gas appliances often have their source of ignition at or near that level.
Earlier efforts, such as those disclosed in U.S. Pat. No. 5,797,355, substantially raised the probability of successful confinement of ignition of spilled flammable substances from typical spillage situations to the inside of the combustion chamber. Other following structures, such as those disclosed in U.S. Pat. Nos. 5,950,573; 6,003,477; 6,082,310; 6,085,699; and 6,085,700, for example, have built on the break through success of ""355.
Although the water heaters described in the above-identified patents have been well received and highly successful with respect to increasing the resistance to ambient flammable vapors, certain portions of the U.S., especially California have stringent low NOx emissions regulations and requirements. We have discovered an ongoing challenge associated with meeting these limits with such structures for the following reasons.
An important element of such flammable vapor resistant water heaters is a flame arrestor or flame trap placed strategically in the air intake opening to the combustion chamber of the water heater. This is accomplished by creating a controlled opening for combustion air and otherwise sealing off the combustion chamber. It was discovered that placement of the flame arrestor relative to the pilot is important in consistently igniting the flammable vapors in different spill scenarios. Both the pilot and main burners must be able to light the flammable vapors soon after they enter the combustion chamber.
There are also preexisting conditions that dictate the relative positioning of the pilot to the burner that it is to ignite. Thus, the design wherein the same pilot that would light the burner would also ignite the flammable vapors, if present, had the effect of setting a relationship of the inlet air relative to the burner. The result was that the air inset for the flame trap is off center and not symmetrical.
Most natural gas burners used in storage water heaters are xe2x80x9cpanxe2x80x9d burners. They are made of sheet metal making them a very economical choice and their flame pattern is well suited for heating a typical storage tank bottom. Such pan burners are designed to operate with air evenly distributed around the inlet to the mixing chamber of the burner. They have the opening to the mixing chamber placed at a specific distance and concentricity from a gas orifice. This allows the gas to flow from the orifice and mix with the combustion air consistently and unencumbered.
However, in that design, the relationship of the flame arrestor to pilot to burner creates imbalances in the availability and locations of primary and secondary air through out the combustion chamber. This imbalance can cause several side effects in the combustion process. It leads to higher levels of NOx production as the flame temperature runs hotter on the non-arrestor side due to lack of excess air to help cool the process. The air at the mixing chamber opening is also distributed unevenly. This also contributes to higher NOx production by disturbing the ratio of primary and secondary air. This can deprive the burner of enough air to mix with the amount of fuel. The fuel and air may also not mix as intended. With the air availability skewed from side to side, the mixture may not be as homogenous as a mixing chamber in a symmetrical environment and, therefore, produce pockets of rich mixture and pockets that are too lean. This has been found to increase NOx production due to having isolated hot spots. It is known that one of the primary factors of NOx production is flame temperature exceeding 2800 degrees F. Formation of thermal NOx increases exponentially with combustion temperature, and increases by a square root relationship with the presence of oxygen in the combustion zone. This excessively rich primary mixture is also more likely to flashback on extinction when using alternate higher BTU input fuels that are required for ANSI testing. (Butane-Air). Accordingly, it has been a primary objective to produce a water heater that simultaneously addresses the issue of resistance to flammable vapors and can meet ever increasingly stringent low NOx emissions regulations and requirements by the various regulatory bodies.
The invention relates to a water heater including a water container and a combustion chamber adjacent the container. The combustion chamber has at least one flame trap to admit air and extraneous fumes into the combustion chamber. The flame trap (sometimes also referred to as an xe2x80x9cair inletxe2x80x9d or xe2x80x9cair inlet platexe2x80x9d) has a plurality of ports. The ports are sized and shaped to cause air and extraneous fumes to pass through the ports, yet confine ignition and combustion of the extraneous fume species within the combustion chamber. The water heater includes a burner associated with the combustion chamber and arranged to combust fuel to heat water in the container. A low NOx draft hood is positioned relative to the burner and the flame trap to enhance combustion dynamics to reduce production of NOx emissions.